Conventional coloured base coat for car body, which are generally made of polyester, melamine, cellulose acetate butyrate resin ("CAB resin") or acrylic melamine resin, hardly have any adhesion to polyolefinic substrates consisting of so-called crystalline polyolefins with low polarity.
For that reason, paints containing modified or copolymerized chlorinated polyolefin containing anhydride are used as colored base coat for the coating on polyolefinic substrates. Though they all have good adhesion to such substrates, these paints are used only for primer coating or 1-coat top coating because of such problems as no hardening, poor adhesion or poor secondary adhesion after washing with water, etc., which are vital problems for coating materials, in the coating made by the method of directly coating a colour base having painting pigment and completing the coating with a single baking, i.e. primerless 2-coat 1-bake system (wet on wet system of coloured base coat and top clear coat).
Moreover, the paints containing modified or copolymerized chlorinated polyolefin containing anhydride which are supplied today are mostly of lacquer type or urethane crosslinking with polyisocyanate and of short working life and, therefore, it was practically very difficult to use them by the primerless 2-coat, 1-bake method.
Furthermore, the coated film obtained had a defect of yellowing regarding long-term weatherability and problems such as poor waterproofing, poor chemical resistance, etc. More specifically, they had a problem of producing dechlorination from polyolefinic substrate and modified chlorinated polyolefin containing anhydride, causing yellowing of aluminium pigment and top coat clear, etc.
As attempts for improving the polyolefinic substrate itself, various trials are being made such as plasma treatment (surface oxidation by oxygen plasma), frame treatment (surface oxidation by frame), ultraviolet ray treatment (furnishing of functionality by ultraviolet ray), etc. as measures for removing trichloroethane (improvement of adhesion. However, those measures also have problems such as unstable adhesion, etc. depending on the kind of polyolefinic substrate (degree of non polarity) and still remain in the stage of study. Moreover, while it is possible to improve adhesion by introducing functional group into the polyolefinic substrate, it makes the substrate costly.
Because of such defects, said formation of coated film is still insufficient in commercial value and has therefore not yet put to practical use. For that reason, the present invention intends to provide a thermosetting coating composition provided with workability, excellent adhesion, weatherability, good appearance, etc. capable of primerless coloured base coat and top clear coat by a 2-coat, 1-bake method on the polyolefinic substrates. Moreover, the present invention intends to provide a coated body having excellent adhesion, weatherability, good appearance, etc. realized by a 2-coat, 1-bake method.